Liquid crystal (LC) displays are now nearly ubiquitous in our culture, being used in both monochrome and color displays in a variety of products from watches to automobile gauges and from road signs to computer displays. It is most desirable that monochrome displays are simply black and white with no particular cast of color. Similarly, it is imperative for quality color displays that all colors be transmitted equally well. If a display is less transmissive for one wavelength compared to another, the display will not show true colors and will be less marketable than a display showing true colors.
LC displays rely on the birefringence (An) of the LC, i.e., the difference in refractive indices between different orientations of the LC. Birefringence, .DELTA.n=n.sub.e -n.sub.o, where n.sub.e is the index of refraction along the extraordinary axis of a birefringent material (parallel to the optic axis) and n.sub.o is the index of refraction its ordinary axis (perpendicular to the optic axis). The optimal thickness of an LC cell such that it behaves as a half-wave plate, to maximize contrast and true color transmission, at a given wavelength is proportional to the birefringence. The optimal birefringence for a fixed pathlength, i.e., thickness of LC, increases with increasing wavelength as shown in FIG. 1. In contrast, birefringence of a given material generally decreases as a function of increasing wavelength (FIG. 1). The change in birefringence of a material as a function of wavelength is called birefringence dispersion. (Herein, the term "positive birefringence dispersion" is used for birefringence that decreases with increasing wavelength and "negative birefringence dispersion" is used for birefringence dispersion that decreases with increasing wavelength.) Thus, if birefringence of an LC cell is optimized for transmission at one wavelength by optimization of cell thickness, it will not be the optimal birefringence at a second wavelength and as a consequence light transmission through the cell at the second wavelength will be lower.
Typically, in designing an LC device, a compromise is made by setting cell thickness for optimal transmission of a wavelength in the middle of the operational wavelength range (i.e., at the design wavelength). For LC devices used in the visible, cell thickness is chosen to optimize transmission of green light, giving a cell less than optimal, but useful, transmission in the red and blue. Such a cell has a slight yellow or green cast.
If the birefringence dispersion of an LC material were negative (increasing in slope as a function of increasing wavelength), cells made from this material would exhibit significantly less chromatic behavior. In general, LC materials, i.e. mesogenic compositions, which exhibit a lower positive (including zero) or negative birefringence dispersion than existing materials will be useful for decreasing the chromatic behavior of LC displays and related electrooptical devices. Such mesogenic materials will be useful in optical filters with improved color balance, larger free spectral range, maintaining high resolution with fewer filter stages and in tunable Fabry-Perot filters using liquid crystal spatial light modulators (SLMs).
Furthermore, ferroelectric liquid crystals (FLCs) used in displays often have quite high birefringence requiring the use of thin cells. When thin LC cells are used, small variances in cell thickness can have a significant effect on the cell's optical properties. For example, a 0.1 .mu.m variance in thickness of a cell that is 1.1 .mu.m thick results in a .+-.9% difference in transmission, while the same variance in a thicker 1.9 .mu.m cell results in only a .+-.5% difference. Thinner LC cells also tend to suffer from non-uniform spacing, which can lead to shorts. Environmental contamination of LC cells, for example by inclusion of dust and other contaminants, has a more severe effect on thinner cells. Designs using thicker cells, for more stability, easier manufacturing and lower cost, require LC materials with generally lower birefringence (compared to presently available materials). There is a general need in the art for LC materials, particularly FLC materials, with decreased birefringence.
Ferroelectric liquid crystals (FLCs) are true fluids possessing thermodynamically stable polar order. As the liquid crystal cools from a normal isotropic liquid to a crystalline state, it passes through a series of mesogenic phases of increasing order. A typical phase sequence includes several phases, of which only the tilted smectic C* (S*.sub.c) phase possesses the thermodynamically stable polar order necessary to exhibit a net dipole moment. In the S*.sub.c phase the molecules self-assemble into layers, with the long axis of the molecules coherently tilted with respect to the layer normal. The single polar axis of the phase is normal to the tilt plane. For most such FLCs, a spontaneous macroscopic dipole density or spontaneous ferroelectric polarization P along the polar axis is easily measurable.
The ferroelectric nature of a C* phase affords a very strong coupling of the molecular orientation with external fields, leading to a high contrast electro-optic light valve with fast response relative to the well known nematic devices currently in use. The complicating factor of the C* helix was solved with the invention of the Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) light valve. In the SSFLC geometry, the helix is spontaneously unwound due to surface interactions with bounding glass plates. In this case, when the director prefers a parallel orientation with respect to the surface plates, two states are allowed. In one state the molecules tilt right by tilt angle .theta., while in the other state they tilt left. In both cases, the ferroelectric polarization vector is pointing normal to the title plane (normal to the surface of the glass plates).
Due to the birefringence of FLC molecules, the two states have different optical characteristics. When the tilt angle .theta.=22.5.degree., and the thickness of the cell is tuned correctly relative to the birefringence, then the cell behaves as a half wave plate, and can be aligned between crossed polarizers such that one state gives good transmission, while the other state shows good extinction, giving rise to the desired electro-optic effect.
SSFLC cells show very high contrast (1,500:1 demonstrated), low switching energy, bistability, high resolution (.congruent.10.sup.7 pixels/cm.sup.2 demonstrated, 10.sup.8 pixels/cm.sup.2 possible) and other performance characteristics which make it particularly attractive for many optoelectronic applications.
Compounds which self-assemble into the smectic C phase are often termed C phase mesogens. While there is currently no detailed understanding of the relationship between molecular structure and the occurrence of LC phases, empirically, C phase mesogens generally possess a rigid core separating two "floppy" tails. The tails of chiral and achiral mesogens can include a variety of chemical functionalities, but components of commercial mixtures often have one or two alkyl or alkoxy tails. The tails often have similar lengths, and both are typically longer than four carbons. Many compounds of this type also exhibit a nematic phase. For C* mesogens generally one of the tails will possess one or more tetrahedral stereocenters.
In order to be useful in the many types of devices of interest, FLC materials must possess properties never achievable in a single compound, but the stable temperature range and other material parameters can in general be tuned by mixing components. Commercial LC mixtures are generally composed of at least eight components. FLC mixtures generally contain two types of components: 1) A smectic C host, designed to afford the required temperature range and other standard LC properties; and 2) Chiral components designed to induce ferroelectric polarization and produce fast switching or other desirable properties (e.g., tilt angle adjustment)in the FLC film. FLC mixture may also contain additional achiral components that adjust other desirable FLC properties.
Birefringence refers to the property of a liquid crystal to interact more strongly with light along one LC axis than along another LC axis. As discussed above, most LCs are made of a core with extensive electron delocalization, to which one or two tails are attached to help orient the molecules, give a dipole moment or polarization or confer other desirable properties on the molecule. Typical LC are rod-shaped with the majority of the pi-electron delocalization along the long or extraordinary axis (also referred to as the director). As a consequence the extraordinary axis of LCs have the higher index of refraction, so their birefringence .DELTA.n=n.sub.e -n.sub.o is positive. Birefringence of a liquid crystal at a given wavelength is: ##EQU1## where .DELTA.n is the birefringence at a given wavelength, G is a constant, T is the temperature, .lambda. is the particular wavelength, .lambda.* is the mean resonance frequency which can be calculated given the spectrum of a material or the its birefringence at several wavelengths. See: S. -T. W (1986) Phys. Rev. A 33:1270; S. -T. W (1987) Opt. Eng. 26:120; S. -T. W, C. -S. W (1989) J. Appl. Phys. 66:5297; S. -T. W et al. (1993) Opt. Eng. 32:1775. As the wavelength of interest moves away from .lambda.*, the birefringence decreases asymptotically until in the infrared, the birefringence is relatively constant (except near IR absorbencies). There is, however, a large amount of birefringence dispersion in the visible spectrum. This is particularly true if .lambda.* is close to the visible region so that .lambda..sup.2 -.lambda.*.sup.2 is small. While the birefringence of the typical LC is higher at short wavelengths than at longer wavelengths, optimization of LC cells as half-waveplates at a given wavelength generally require the opposite behavior of birefringence as a function of wavelength. FLC cell half-wave plates in particular require: ##EQU2## where a wavelength (.lambda.) of about 500 nm is usually chosen as the optimal for LC cells (for applications in the visible). As indicated in FIG. 1, this thickness is then not optimal for all wavelengths of visible light due to the birefringence dispersion.
FIG. 2 shows transmission (measured and calculated) at different wavelengths for cells with three different FLC materials (a standard mixture and two theoretical mixtures), normalized for 100% transmission of 500 nm light. The first measured transmission (solid line) is ZLI 3654 which has typical birefringence behavior with a .lambda.* of 217 nm. With this mixture, only about half of the 400 nm light and about 70% of the 700 nm is transmitted. A cell using this material has a noticeable greenish cast. The second, a calculated transmission (dotted line) is that based on use of a theoretical material in which the birefringence is invariant with changing wavelength. A cell using such a material is calculated to transmit about 83% of 400 nm light and about 83% of 700 nm light. Such a cell would have much truer color, particularly in the blue, compared to the standard FLC cell. The third, another calculated transmission (dashed line) is that based on use of a theoretical material in which the birefringence dispersion is negative, with the absolute value of the proportional change as a function of wavelength the same as for the standard LC mixture. A cell using such a material is calculated to transmit very nearly 100% of blue light and 92% of red light, resulting in a cell with quite true colors.
The present invention relates in one aspect to low birefringence mesogens or to mesogens having anomalous birefringence dispersion. As used herein, "anomalous" refers to birefringence dispersion atypical for liquid crystal material, either exhibiting zero (as illustrated in FIG. 2 dotted line) or negative birefringence dispersion (increasing with increasing wavelength) (as illustrated in FIG. 2 dashed line), or significantly less positive birefringence dispersion compared to known LC materials. Compounds of this invention can be mesogens or can be employed as components in mesogenic compositions to lower birefringence or to lower birefringence dispersion.
Mesogenic materials of the present invention are chiral nonracemic and achiral materials having mesogenic phases (LC phases) useful in electrooptical devices, including chiral and achiral tilted smectic phase materials (particularly smectic C and smectic A materials) and nematic phase materials. Mesogenic materials of this invention include those that are ferroelectric liquid crystals (FLCs), nematic liquid crystals, and materials useful in SSFLC, electroclinic and DHF devices.
While the discussion of anomalous birefringence dispersion herein has focused on FLC's used in SSFLC devices, other electrooptic devices employing LCs, such as nematic cells will also benefit from the use of LC's with low or negative birefringence dispersion (and more generally from LCs with low or negative birefringence). The mechanism of FLC and nematic cells differ, but in both cases optimization of cell thickness depends on the wavelength of light being transmitted. In nematic cells some efforts have been made to make cells achromatic. For example, a combination of at least two polymer retarder films can be employed as compensators to give light that is reasonably achromatic (T. Scheffer, J. Nehring (1995) SID Seminar Lecture Notes, Vol. 1, M2). However, the use of such external means of chromaticity compensation can be detrimental to contrast ratio or viewing angle of the device. Thus, nematic liquid crystals with little or no birefringence dispersion would be quite beneficial.
Development of methods for creation of organic thin films with large .chi..sup.(2) is a problem of great interest due to the potential utility of such films in the fabrication of fast integrated electro-optic (EO) modulators. Such modulators are hybrid devices wherein the organic material must work in concert with semiconductor integrated circuits. For this application the material design and synthesis task involves three key considerations:
1) Molecules with large molecular second order susceptibility .chi. must be created; 2) The molecules must be assembled into a material with the correct supermolecular stereochemistry to afford the required bulk .chi..sup.(2) ; and 3) This material must be integrated with a semiconductor device--a key process requiring supermolecular stereocontrol on a more global level.
Early in the development of chiral smectic FLC chemistry and physics, the spontaneous thermodynamically stable polar supermolecular structure exhibited by these anisotropic liquids suggested their potential utility in second order nonlinear optics applications. This work, however, showed that FLCs known at the time, such as DOBAMBC, exhibited values of .chi..sup.(2) too small to be useful (d.sub.eff .about.0.001 pm/V for SHG from 1064 nm light). Efforts directed toward design of FLCs with increased .chi..sup.(2) provided materials with demonstrated EO coefficients on the order of 1 pm/V for modulation of 633 nm light at 100 MHz and d coefficients on the order of 5 pm/V for SHG (Arnett et al. (1995) "Technique for Measuring Electronic-Based Electro-Optic Coefficients for Ferroelectric Liquid Crystals" in Thin Films for Integrated Optics Applications, Wessels et al. (eds), Materials Research Society (Pittsburg, Pa.); Walba et al. (1991) Ferroelectrics 121:247; Schmitt, K. et al. (1993) Liq. Cryst. 14:1735). Still, further increases in the magnitude of .chi..sup.(2) are required, since EO coefficients on the order of 50 pm/V are desirable for fast integrated optics applications (Walba, D. M. (1995) Science 270:250). Achieving such a large increase in .chi..sup.(2), however, seems problematical in FLCs since functional arrays with large molecular susceptibility .beta. are typically "long," and tend to orient along the director when incorporated into traditional thermotropic liquid crystal (LC) structures. Since the FLC polar axis is normal to the director, small values of .chi..sup.(2) result as observed for DOBAMBC.
Achieving large .chi..sup.(2) in FLCs involves orientation of "large .beta." functional arrays, typically possessing two rings and a conjugating spacer unit, along the polar axis with a high degree of supermolecular stereocontrol (Williams, D. J. (1984) Angew. Chem. Int.
Ed. Engl. 23:690; Quantum Chemical Computational calculations of Nonlinear Susceptibilities of Organic Materials; Bredas, J. L. et al.(eds.); Special Issue of Mol. Cryst. Liq. Cryst. Sci. Technol. Sect. B, Gordon & Breach (1994) 6(3-4):135; Kanis et al. (1994) Chem. Rev. 94:195; Meyers et al. (1994) J. Am. Chem. Soc. 116:10703.) Excellent supermolecular stereocontrol is indeed achievable in FLCs (on the order of 60% polar excess has been demonstrated as evidenced by ferroelectric polarization measurements), and "large d" functional arrays are easily incorporated into LC structures (Kobayashi et al. (1990) Mol. Cryst. Liq. Cryst. Letts. 7:105; Fouquey et al. (1987) J. Chem. Soc. Chem. Comm. 1424; Berdague et al. (1993) Liq. Cryst. 14:667; Ikeda et al. (1993) Nature 361:428; Sasaki et al (1994) J. Am. Chem. Soc. 116:625). However, in all know cases such functional arrays orient along the liquid crystal director n, while the FLC polar axis is normal to the director.
This invention relates in a second aspect to LC compounds for NLO applications. While an LC mesogen is not required for NLO FLCs (doping of achiral C phase hosts with appropriate non-mesogenic quests delivers the required supermolecular stereocontrol) mesogenicity is desirable since for NLO applications achieving the highest possible concentration of NLO active units is advantageous. Certain compounds of this invention that have negative birefringence also exhibit NLO properties. Herein we demonstrate an approach for achieving large .chi..sup.(2) in FLCs with examples of a class of chiral smectic materials demonstrating orientation of large .beta. NLO chromophores along the polar axis.
WO 92/20058 (Walba et al.) published Nov. 12, 1992 relates to ferroelectric liquid crystal materials for nonlinear optics applications. Certain of the compounds reported therein are monomers for the side-by-side dimer materials of this invention. WO 92/20058 takes priority from U.S. patent application Ser. No. 07/690,633 filed Apr. 24, 1991 (now abandoned). U.S. patent application Ser. No. 08/137,093, filed Oct. 18, 1993, (now allowed) is the U.S. national stage application of WO 92/20058. WO 92/20058, U.S. Ser. No. 07/690,633 and U.S. Ser. No. 08/137,093 are incorporated in their entirety by reference herein.
The following U.S. patents provide general descriptions of LC's for electrooptical applications, including FLCs: U.S. Pat. Nos. 5,051,506, 5,061,814, 5,167,855, 5,178,791, 5,178,793, 5,180,520, 5,271,864, 5,380,460, 5,422,037, 5,453,218, and 5,457,235. These patents are incorporated by reference in their entirety herein and provide methods of synthesis for a variety of LC materials, including methods of synthesis of a variety of LC cores and chiral and achiral LC tails that are used in the compounds of this invention. These patents also provide a general description of the properties of LC materials for electrooptical applications, particularly those for use in SSFLC, electroclinic, DHF and nematic devices.